1. Field of the Invention
The present invention relates to helical scan tape recorders and, more specifically, to an apparatus and method for appending data to data previously established on the tape in such a recorder so that the spacing between the appended data and the previously established data is within a defined range.
2. Description of the Related Art
The typical helical scan tape recorder includes a tape onto which data can be written or from which data that has been previously written on the tape can be read using a rotating scanner that has one or more read/write (R/W) heads that rotate about the axis of rotation of the scanner. The tape and the R/W heads are positioned with respect to one another such that the plane of rotation of the R/W heads is at an angle to the longitudinal axis of the tape. Associated with both the tape and the scanner are servo devices for controlling the movement of each. More specifically, there is a tape servo, which is hereinafter referred to as the capstan servo, that is used to control the velocity and position of the tape relative to the R/W heads and a scanner servo that controls the rotational velocity of the R/W heads.
To write data on the tape in the typical helical scan recorder, the capstan servo is used to move the tape at a substantial constant velocity relative to the R/W heads and the scanner servo is used to rotate the R/W heads at a substantial constant angular velocity. This and the orientation of the R/W heads results in the R/W heads each traversing a path over the tape that is at an angle to the longitudinal axis of the tape. As a R/W head traverses this path, a track or swipe of data is established on the tape with the track having a center line that is at an angle to the longitudinal axis of the tape. Due to the substantially constant velocity of the tape and the substantially constant angular velocity of the R/W heads, the swipes are laid down on the tape such that the distance between center lines of consecutive swipes, as measured along the longitudinal axis of the tape is, also, substantially constant.
Once data has been established on the tape in the typical helical scan tape recorder, the data can subsequently be read by using the capstan servo to move the tape relative to the R/W heads at a substantially constant velocity and using the scanner servo to rotate the R/W heads at a substantial constant angular velocity as was done during the write operation. However, even though the tape and the scanner are moving at the appropriate velocities, the paths being traversed by the R/W heads over the tape may be parallel, but not aligned, with the center lines of the swipes established on the tape. Stated another way, the tape and the R/W heads may not be in phase with one another. This phase difference can result in failure to accurately read the data established on the tape. To bring the tape and the R/W heads into phase with one another so that the swipes can be read, the velocity of the tape or the rotational velocity of the R/W heads must be momentarily increased or decreased, as the case may be, using either, or both, the capstan servo and the scanner servo. Generally, the capstan servo device is preferred for making phase adjustments over the scanner servo because the relatively small mass of the tape relative to the scanner makes it easier to control the velocity of the tape. Typically, the data error rate, i.e., the errors detected in reading the swipes, serves as a good indicator that the tape and the R/W heads are out of phase with one another and, as a consequence, is typically used in making the appropriate phase adjustment.
Presently, helical scan tape recorders are generally used to record a large volume of data where the data is provided at a particular frequency or rate and typically results in most, if not all, of the usable space on the tape being employed to record the data. In such situations, the write and read operations described above work quite well. However, there are many applications that require considerably less than all or substantially all of the tape in order to record the data presented. An example of such an application is oil field logging where a device associated with an oil well records production data for the oil well over a defined period and then transmits the data to a receiver, generally located in an airplane passing over the oil well, where it is recorded. Typically, the airplane containing the receiver flies over and records the data from the devices associated with several oil wells that are distantly located from one another. Consequently, this application requires that the recording device be activated to record the data from one oil well, deactivated after the data for the one oil well is recorded, and then reactivated to record the data for the next oil well. Presently known helical scan tape recorders are not feasible in such an application because they cannot reliably record data that is not provided at a particular frequency or rate, i.e., interrupted or provided in a non-continuous fashion. More specifically, if the flow of data is not provided at the required rate, then presently known helical scan tape recorders will not establish the required spacing between adjacent swipes. This may, in turn, compromise data integrity. For example, if a swipe is laid down too close to the swipe previously established on the tape, then the data associated with the swipe previously established on the tape may be destroyed. Stated yet a different way, presently known helical scan tape recorders are incapable of what is typically termed an append operation in the mass data storage industry.
Based on the foregoing, the present invention seeks to address the failing of the presently known helical scan tape recorders in not providing the ability to append data.